The invention concerns a device to symmetrize the radiation of one or several linear optical emitters, in particular wideband laser diodes, whose radiation can be described as light beams with telecentric chief rays infinitesimally tightly packed in an x-direction perpendicular to its direction of emission, or to a main optical axis z, and whose radiation is asymmetrical with respect to the x-direction and a y-direction perpendicular to the x-direction and to the z-direction.
In the case of asymmetric optical emitters, the problem occurs that the output radiation is also asymmetrical. For a large number of applications of optical emitters of this kind it is desirable to have an output that is as symmetrical as possible. For this reason, devices to symmetrize the radiation of asymmetrical optical emitters are needed.
From DE 196 45 150 a device to symmetrize the radiation of a light source built up of a plurality of separate emitters is known. In the case of the light source, it is a laser diode bar which contains a plurality of individual laser diodes. The symmetrizing device consists of a cylindrical lens rotated about the optical axis, a directional optical unit to diffract the light beams from the individual laser diodes, a redirecting optical unit to compensate for the diffraction of the directional optical unit, and a subsequent collimating optical unit.
From DE 198 20 154 a device to symmetrize the radiation of one or several laser diode bars is known, which comprises a cylindrical lens optical system, two continuous angle transform elements with intervening Fourier transform arrays to redirect the light beam and a focusing device. The disadvantage is the high cost of producing the angle transform array.
In addition to the laser diode bars known from DE 196 45 150 and DE 198 20 154, what are known as wideband laser diodes, which are laser diodes with a single, wide emitter, are becoming increasingly important. The geometric dimensions of the radiating surface of the emitter is typically in a range between 50 xcexcmxc3x971 xcexcm up to about 500 xcexcmxc3x971 xcexcm.
The output radiation of these types of wideband laser diodes is extremely asymmetrical. In a first plane (slow axis), which is formed from one axis in the direction of the broad dimension of the emitting surface and one axis in the direction of emission, the divergence of the output radiation corresponds to a numerical aperture of about 0.1. In a second plane (fast axis) perpendicular to the first plane, the output radiation has a considerably greater divergence, corresponding to a numerical aperture of about 0.5.
Because of the different divergences and dimensions in the two planes, the beam quality of the output radiation, and consequently the focusability in the two planes, is very different. The beam product, which is defined as the product of the emitting surface and the divergence of the output radiation, can be used as the measure of the beam quality. The ratio of the beam products of slow axis to fast axis, depending on the width of the emitting surface, is in the range up to about 1:100 for wideband laser diodes. The use of wideband laser diodes consequently requires the use of optical systems to symmetrize the radiation.
From WO 96/02013 a device to symmetrize the radiation of wideband laser diodes is known which, with the aid of a prism system, splits the already collimated output beam of a wideband laser diode along its broad dimension into several individual beams and stacks these beams one above the other. The disadvantage in the case of this symmetrizing device is the complexity of the prism array and the absence of any potential for miniaturization.
From WO 95/15510 another device to symmetrize the radiation of wideband laser diodes is known, in which the output radiation of the laser diode passes through a system of two highly reflective surfaces barely tilted towards each other in such a way that at the output of the device the result is a symmetrized reconfiguration of the laser diode beam. The disadvantage with this device is the high cost of adjusting the entire system and the great expense that would be associated with miniaturizing the system.
Starting from the disadvantages of the prior art, the object of the invention is to provide a device to symmetrize the radiation of linear optical emitters, consisting of micro-optic components which are relatively simple to manufacture, and which is amenable to economical miniaturization. Furthermore, beam density losses associated with symmetrizing should be kept as low as possible, and good imaging characteristics should be ensured. The object is furthermore to provide arrays and applications for devices of this kind.
The output radiation of one or several linear optical emitters, i.e. in-line emitters, can be described by a linear array of individual light beams with telecentric chief rays, lying infinitesimally close together at least in one or several light beam groups, in a perpendicular direction to the direction of emission and thus also in a direction x perpendicular to an optical main axis of the optical unit z. A single light beam group can have beams from each one of the emitters, if necessary in a device with several emitters and/or individual beams from several emitters. Consequently there can be a space between each of the light beam groups in the x-direction and/or y-direction.
Under the invention, a device to symmetrize the output radiation is proposed, which has a cylindrical lens optical system, which can contain one or several cylindrical lenses per emitter or possibly for several emitters jointly. The cylindrical lenses bring about a collimation of each beam in the y-direction. At least one of the cylindrical lenses can be rotated about the direction of emission, in order to diffract each beam at different angles in the y-direction. As an alternative to rotating at least one of the cylindrical lenses, the diffraction in the y-direction can be achieved by means of a separate discontinuous diffracting element.
The device under the invention further contains a director-collimator optical unit positioned along the main optical axis z, which collimates each beam in the x-direction and diffracts them at different diffraction angles in the x-direction and the y-direction. The director-collimator optical unit can also be positioned in a direction perpendicular to the main optical z-axis, e.g. x- or y-direction, offset to the main optical axis. The diffraction takes place in such a way that the chief rays of the individual beams of one beam group converge in the x-direction at a specified distance from the emitter(s) and run parallel in the y-direction.
Finally, the device under the invention has a redirecting optical unit, which can also be positioned along the z-axis. This diffracts the light beam in the x-axis. Advantageously it compensates for the diffraction of the light beam in the x-direction caused by the director-collimator optical unit, as the result of which the beam product is reduced in the xz-plane, while it is increased in the yz-plane. The asymmetrical output radiation of the linear emitter is thus transformed into largely collimated radiation with an approximately rectangular or square cross section. The beam products in the yz-plane and in the xz-plane are conformed in this way.
The device under the invention to symmetrize the radiation of a linear optical emitter, compared with the known devices, has the advantage that it consists only of assemblies which can be manufactured and miniaturized economically. The device is also relatively uncomplicated to set up. Compared with the known devices to symmetrize the radiation of a laser-diode bar, the invention shows a clear reduction in aberrations, which can be attributed to the superior arrangement of collimating, directional and redirecting optical devices.
The present invention comprises, in addition to the devices to symmetrize radiation, arrays of several devices of this kind, where the radiations emanating from one of the devices are superposed on one another, polarization-coupled or wavelength-coupled. For this, a xcex/2 delay plate can be positioned following the particular redirecting optical unit in at least one of the optical paths as a polarization-rotating element. Furthermore, the particular partial rays of the individual devices are superposed on one another by means of a mirror element. The mirror element can be polarization-selective or wavelength-selective. In similar fashion, it is also possible to superpose individual light beam groups from a single device under the invention on one another, polarization-coupled or wavelength-coupled.
These kinds of arrays of several devices under the invention are particularly suitable for generating specific properties of the received conformed beam, for example, a non-polarized output beam, although the output radiation of individual wideband laser diodes is polarized. The intensity of the light beam received in the end can also be varied by this means.
The device under the invention and the arrays of such devices under the invention can be used directly to generate a desired radiation, or also indirectly to pump lasers, in particular in the fields of printing and photographic technology, for micromaterial processing, in the field of medical technology, in telecommunications technology or in lighting and display technology. They also have an application in the field of analytics.